Dissolve the sulphite in 200cc of water warmed to 52C or 125F thoroughly and allow it to cool. Then add the acid very slowly with constant stirring.

Dissolve the alum in 600cc of water warmed to 52C or 125F. Allow it to cool to below 20C (68F).

Pour the alum solution slowly and carefully INTO the acid solution, not the other way around. Top off with water to attain your 1 litre volume.

WORKING SOLUTION
Dissolve 200g of Sodium thiosulphate (hypo), crystal, into 500cc of warm (52C or 125F) water. Allow to cool and then add 125cc stock hardening solution into it and then top it off with water to attain a volumne of 1 litre.